Radio networks usually operate in such a way that the signalling required for establishing calls (as well as for the registration of MSs with the system) is performed on a control channel and the speech or data communication during the calls is conducted on traffic channels. This requires each base station to have more than one channel, since at least one channel is needed as a control channel and at least one channel as a traffic channel. Typically one control channel and several traffic channels are provided. In some radio networks, a less complicated operation is obtained by utilizing a single channel for both the control signalling and the speech communication. Furthermore, radio networks with very low traffic levels may be single-channel systems.
Radio traffic can be of three different types: 1) simplex, 2) semiduplex or 3) duplex.
In simplex operation, mobile stations MS employ the same frequency for traffic in both directions. Thus, when one party is speaking (and presses the transmission switch or pressel), the other is listening. During listening, the receiver and the loudspeaker of the listening party are muted. The simplest radio systems operate in this way. If two or more mobile stations communicate with one another without any base station or any other radio system, the mobile stations operate in the simplex mode.
The semiduplex mode utilizes for the traffic two frequencies, i.e. one frequency pair. The transmitters of both mobile stations (or in a group call, all mobile stations) are tuned to one frequency and the receivers to the other frequency. Naturally, these frequencies are reversed at the base station (i.e. the transmitting frequency of the mobile station is the receiving frequency of the base station, and vice versa). Yet also in this case it is impossible for the transmitting and receiving subscriber to speak simultaneously. The receiver and the loudspeaker of the transmitting mobile station are muted during the transmission. The semiduplex mode enables the use of fixed stations, such as telephone or exchange interfaces or special control points. A control point (CP) is a more or less conventional telephone set connected to an exchange and may be used for performing various standby tasks. The semiduplex mode allows effective utilization of the radio frequencies, since within the area of one base station two or more mobile stations may use the same frequency pair.
The operation of a duplex network is similar to that of a normal telephone network, that is, both subscribers can speak and hear one another for the entire duration of the call. Such networks include public land mobile networks, for instance. In a network of this kind, two pairs of channels must be reserved for a call between two mobile stations.
Semiduplex-type radio networks (networks in which mobile station users cannot speak simultaneously during a call) are always attended by the problem of ensuring that call release is received if the system clears down the call (for instance in order to allocate the channel for an emergency call). If one of the subscribers is transmitting during the release operation, the pertinent mobile station is not capable of receiving and hence will not receive the release message. In consequence, the mobile station remains on the traffic channel and participates in the next call as well.
One has generally attempted to solve this problem in trunked networks (networks in accordance with the MPT 1327 and 1343 specifications, for instance) by sending a special release message at the beginning of each call, commanding all unintended parties possibly participating in the call to leave the traffic channel. This is a relatively feasible solution as such, even though a subscriber can, by pressing the pressel (transmission switch) for an extended period, succeed in bypassing receipt of these messages and remain with the call even thereafter. For this reason, the special release messages are normally repeated later during the call. A drawback with this procedure is that part of the capacity of the traffic channel must be used for transmission of traffic that is unnecessary for the call, and in practice this also disturbs the telephone users.